The present invention relates to an internal combustion engine.
In recent years, more and more strict regulations for control of exhaust gas from automobile has been enforced under strong demands on maintenance of better environmental conditions. In particular, for diesel vehicles, strict regulations have been enforced on NOx (nitrogen oxides) and soot (particulate matter) emissions in exhaust gas.
Conventionally, NOx and soot emissions in the exhaust gas are reduced, for example, by a lean premixed compression ignition method in which the timing of fuel injection is set earlier to prolong ignition delay. According to this method, fuel is burnt at low temperature since it is turned into lean gas mixture after completion of injection thereof. Thus, NOx and soot emissions in the exhaust gas can be reduced to substantially zero.
As a further conventional way to reduce NOx emission in exhaust gas, an EGR (Exhaust Gas Recirculation) system is known in which engine exhaust gas is partly recirculated as EGR gas together with intake air to the engine. The EGR gas has relatively high specific heat and can absorb a large amount of heat. As a result, the more the amount of the EGR gas is increased, i.e. the more the EGR ratio (amount of EGR gas/(amount of EGR gas+amount of intake air)) is increased, the more combustion temperature in combustion chamber is decreased and the more the amount of NOx produced is decreased.
A further conventional way to suppress NOx emission in exhaust gas is to use NOx catalyst which promotes decomposition of NOx into harmless gas.
Particulate matter in exhaust gas is conventionally reduced by using a diesel particulate filtering unit with a particulate filter therein (DPF) or a NO2 regeneration type diesel particulate filtering unit (catalytic DPF).
The above-mentioned lean premixed compression ignition method may almost completely eliminate NOx and soot emissions at low to medium engine load; the amount of HC will, however, increase due to the fuel injection at earlier timing. Moreover, earlier ignition disadvantageously leads to higher fuel consumption. Furthermore, the method can hardly provide stable operation at high engine load because of possible abnormal combustion such as knocking.
In the case of the EGR system, NOx emission is reduced as the EGR ratio is increased; however, when the EGR ratio is increased beyond a certain limit, the amount of soot produced rapidly increases.
In the case of NOx catalyst being used, such NOx catalyst cannot fulfill its function completely and therefore is not enough for practical use.
Use of DPF or catalytic DPF may contribute to removal of particulate matter; but it has nothing to do with reduction of NOx emission at all.
To solve the above and other problems, it is an object of the invention to provide an improved internal combustion engine such as diesel engine which can reliably reduce NOx and particulate matter emissions in engine exhaust gas over all engine-operating load range from low to high load and which can also reduce fuel consumption.
The internal combustion engine according to the invention comprises means for decreasing combustion temperature and means for increasing ignition delay, both of said means being used at low to medium engine load such that combustion of fuel in the engine is at a temperature lower than that at which NOx is produced and at an equivalent ratio lower than that at which soot is produced, said means for decreasing combustion temperature being used at medium to high engine load such that combustion of the fuel in the engine is at a temperature lower than that at which NOx or soot is produced.
The internal combustion engine according to the invention may further comprise an exhaust gas recirculation passage for communication of an exhaust conduit downstream of a turbine of a turbocharger with an intake conduit upstream of a compressor of the turbocharger, and means for removing particulate matter provided on a portion of the exhaust conduit upstream of a connection of said exhaust conduit with the exhaust gas recirculation passage.
The means for increasing ignition delay may comprise means for controlling timing of fuel injection to set earlier timing for fuel injection.
The means for decreasing combustion temperature may comprise means for recirculating exhaust gas to the engine via the exhaust gas recirculation passage.
Circulation ratio of the exhaust gas to be circulated to the engine via the exhaust gas recirculation passage may be set to more than about 40% at low to medium engine load, to more than about 50% at medium to high engine load and to less than about 50% at high engine load.
The internal combustion engine of the invention may further comprise means for cooling the exhaust gas in the exhaust gas recirculation passage.
As the engine load is increased, timing to close an intake valve for performing intake to the engine may be retarded by control means for adjusting said timing for closing.
Thus, according to the invention, it is ensured that NOx and soot emissions from the engine can be reduced over the entire engine-operating load range from low to high load and that fuel consumption can be also reduced.